The green anaconda can occasionally reach lengths of more than 6.6m as an adult. However, a more common maximum length in mature snakes is 5m in females and 3m in males. Their weights are not so well known, due to the difficulty in measuring them, though they have been reported to weigh around 30kg to 70kg as average-sized adults. Although not as long as the reticulated python, it is heavier, and probably the heaviest squamate in the world, possibly only beaten by the Komodo dragon. Reports of 35-40 feet (~10.67-12.2 metres) and even longer individuals also exist, but no specimens with such sizes have ever been deposited at museums, and there isn't much evidence to support them. It has been noted that a 4.5m green anaconda's weight in comparable to that of a 7.4m reticulated python, which has much less bulk than the anaconda.

The colour pattern consists of an olive-green background with black blotches along the body. The head is narrow compared to the body (in contrast with pythons, where the head widens), often with distinctive orange or yellow striping on each side. The eyes and nostrils are set high up on the head, allowing the snake to see out of the water without exposing its massive body to potential prey.

An ambush predator, the green anaconda is an obligate carnivore, and will feed on almost any animal that crosses its territory. It uses water to hide from prey, while waiting for a meal to come by. It will then strike at the prey (usually near the neck or torso), gripping onto it with its jaws, and wrap its coils around the body, slowly tightening around it and killing the animal by constriction.

Anacondas are capable of swallowing nearly any animal they can overpower, including birds, a variety of mammals, and other reptiles. Extremely large specimens may consume even larger prey such as tapirs, deer, capybaras, and small caimans. Such prey is not regularly taken, however. Less often, they eat fish. Cannibalism is also common, and in most cases involves a large female eating a smaller male, including her own partner. In addition, rumours exist that they will snatch humans and jaguars, though there are no verified reports to support these claims. Theoretically speaking, however, it is possible for an excessively large anaconda to swallow a child or small adults, as the shoulders are of insufficient width in these humans to prevent ingestion.